Which is least thermally stable?

Options

A. `Li_(2)CO_(3)`
B. `MgCO_(3)`
C. `BaCO_(3)`
D. `BeCO_(3)`

Explanation

To determine which carbonate among the alkaline earth metal carbonates is the least thermally stable, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Thermal Stability of Carbonates**: The thermal stability of carbonates generally depends on the size of the cation. Larger cations tend to stabilize the carbonate structure better than smaller cations.

2. **Identify the Cations**: The alkaline earth metals include:
- Lithium (Li⁺)
- Beryllium (Be²⁺)
- Magnesium (Mg²⁺)
- Calcium (Ca²⁺)
- Strontium (Sr²⁺)
- Barium (Ba²⁺)

3. **Compare Cation Sizes**: The ionic sizes of the cations increase as you move down the group in the periodic table. The order of ionic sizes from smallest to largest is:
- Be²⁺ < Mg²⁺ < Ca²⁺ < Sr²⁺ < Ba²⁺ < Li⁺

4. **Determine the Least Stable Carbonate**: Since smaller cations lead to less thermal stability in their carbonates, we need to identify the smallest cation among the alkaline earth metals. Beryllium (Be²⁺) is the smallest cation in this group.

5. **Conclusion**: Therefore, beryllium carbonate (BeCO₃) will be the least thermally stable carbonate among the alkaline earth metal carbonates.

### Final Answer:
The least thermally stable carbonate is **Beryllium Carbonate (BeCO₃)**.

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